Open center hydraulic circuits use pumps which supply a continuous flow. The flow is returned to tank through a control valve's open center; that is, when the control valve spool is centered, it provides an open return path to tank and the fluid is not pumped to a high pressure. Otherwise, if the control valve is actuated, it routes fluid to and from an actuator and tank. The fluid's pressure will rise to meet any resistance, because the pump has a constant output. If the pressure rises too high, fluid returns to tank through a pressure relief valve. Multiple control valves may be stacked in series. This type of circuit typically uses inexpensive, constant displacement pumps.